A number of lymphocyte surface proteins are anchored in the cell membrane by glycophosphatidyl inositol (known as GPI) linkages instead of hydrophobic protein domains. Treatment of mouse T lymphocytes with antibodies specific for two such proteins, Thy-1 and Ly-6, are known to induce proliferation. We have found that antibodies specific for Qa-2, a GPI-anchored class I histocompatibility antigen, can also activate mouse T cells. To determine whether the GPI-anchor is important for this pathway of cell activation, we produced transgenic mice expressing either normal GPI-anchored Qa-2, or Qa-2 molecules with a membrane-spanning protein domain derived from H-2. Our studies show that only lymphocytes from transgenic mice carrying GPI-anchored forms of Qa-2 can be activated in vitro by Qa-2-specific antibodies. We also show that transgenic mouse T cells expressing a GPI-anchored form of H-2Db can be activated by anti-H-2Db antibodies. These results strongly indicate that the GPI-anchor is critical for this pathway of T cell activation.

DNA structural analysis of the Qa region in two BALB/c mouse substrains with different Qa-2 phenotypes reveals that a deletion of DNA has occurred in BALB/cBy (Qa-2-) mice relative to BALB/c (Qa-2+) mice. We propose that this deletion arises from unequal crossing-over and recombination between adjacent BALB/c class I genes and results in the generation of a hybrid class I gene in BALB/cBy mice. Furthermore, we suggest that this is a direct cause of the change in Qa-2 phenotype. Further support for this model was obtained from transfection experiments in which cloned genes from the equivalent part of the Qa region in C57BL/10 mice were introduced into L cells. Four C57BL/10 genes, arranged in two almost identical pairs, encode polypeptides that are precipitated from lysates of transfectants with anti-Qa-2/3 antiserum. Although loss of one pair of these genes in BALB/c mice has no qualitative effect on Qa-2 phenotype, the loss of both pairs of genes via gene fusion leads to the loss of the Qa-2+

We have studied tolerance induction in transgenic CBA mice expressing H-2Kb genes under the influence of guinea-pig alpha-lactalbumin (KAL) or human beta-globin gene promoter (K beta). KAL radio-resistant cells, but not bone marrow derived cells, induce tolerance to H-2Kb in chimeric mice. In contrast, bone marrow derived and radio-resistant cells of K beta mice induce tolerance. Although appropriate, tissue-specific, expression of H-2Kb molecules occurs in KAL and K beta mice, H-2Kb is expressed at low levels in thymus of transgenic mice. In addition, dendritic cells and macrophages express H-2Kb molecules when K beta, but not when KAL bone marrow is cultured in vitro. The mode of tolerance induction was examined in double transgenic mice by mating KAL or K beta mice to mice expressing TCR transgenes (Tg-TCR) derived from a H-2Kb specific, CD8-independent cytotoxic T cell clone. In both cases, a large number of Tg-TCR+ CD8+CD4+ thymocytes develop but mature CD8+CD4- thymocytes fail to appear suggesting that thymocytes are eliminated late in development. Some CD8-CD4- and CD8-CD4+ Tg-TCR+ T cells develop in double transgenic mice and respond to activation through their TCR-CD3 complex in vitro, although no responses to stimulation with H-2Kb expressing cells were detected. Thus, tolerance induction in KAL and K beta mice proceeds via a deletional mechanism that is inefficient due either to low numbers of H-2Kb expressing thymic cells or to the low levels of H-2Kb expressed by thymic cells, or to a combination of these factors.

The influence of microenvironment on the course of CD8 + T cell responses in vivo was investigated by injecting H-2Kb-specific T cells from donor TCR transgenic (TCR-Tg) mice into H-2kb-Tg mice. H-2Kb expression in recipients was either ubiquitous (CBK mice) or restricted to myeloid and erythroid cells (K beta mice). Donor T cells proliferated as extensively and acquired similar surface phenotypes in spleen of both recipient types. Thus, neither the restricted pattern of H-2Kb expression nor the significantly reduced level of H-2Kb expression by myeloid cells in Kbeta recipients affects the ability of the splenic microenvironment to prime T cell proliferation in vivo. However, an unsustained burst of cytolytic activity was generated rapidly in spleen of CBK recipients, whereas relatively little cytolytic activity was generated in K beta spleen. This indicates that effector T cells were not generated efficiently in spleen of Kbeta recipients even though extensive T cell proliferation was taking place in this microenvironment. Furthermore, activated donor T cells dispersed rapidly throughout primary and secondary lymphoid organs of Kbeta recipients, whereas few T cells migrated from spleen in CBK recipients. Consequently, the course of CD8+ T cell responses and the anatomical distribution of activated T cells are profoundly influenced by the nature of the antigenic microenvironment encountered in vivo. We conclude that T cells rapidly proliferate and acquire new tissue-homing characteristics but do not differentiate into cytolytic effector cells at the site of priming when they encounter myeloid cells expressing low levels of antigen in vivo.

HLA-G, a human MHC class I molecule expressed on the trophoblast during pregnancy, was expressed in transgenic mice by recombining the HLA-G gene with a transcriptional promoter from a murine H-2 MHC class I gene. Skin grafts from HLA-G transgenic mice were rejected by non-transgenic mice showing that HLA-G behaves as a xenotransplantation antigen in mice. Further investigation revealed that murine T cells recognize native HLA-G directly as a xenoantigen or they recognize processed peptides derived from HLA-G presented in the context of murine MHC molecules. HLA-G molecules also function as restriction elements capable of presenting peptides to murine T cells since immunization of HLA-G transgenic mice with peptide that binds specifically to HLA-G molecules elicited HLA-G-restricted, cytotoxic T cell responses. In addition, murine T cell responses to human xenoantigens are enhanced when responder cells originated from HLA-G transgenic mice. Based on these observations, we conclude that expression of HLA-G molecules influences selection of the murine T cell repertoire and that HLA-G exhibits immunological properties that are indistinguishable from classical HLA class I molecules when expressed in transgenic mice. Thus, any unique immunological functions mediated by HLA-G must arise from the distinctive, trophoblast-specific pattern of HLA-G expression in humans and not from structural peculiarities of HLA-G molecules.